In a fabrication process of a semiconductor device, a substrate processing apparatus, which is configured to rotate a wafer, is used to clean and/or dry the wafer. In order to obtain the same results in processing of multiple wafers, these wafers are processed under the same process conditions. For example, in a wafer cleaning process, each wafer is rotated at the same speed, while the wafer is supplied with a cleaning liquid at the same flow rate. In this state, the wafer is cleaned with a cleaning tool, such as a sponge or brush.
A thickness and a multilayer structure vary from wafer to wafer. Such a variation in wafer structure may affect a manner of vibration of a wafer surface when a wafer is being rotated. As a result, the processing results obtained may vary. More specifically, due to the vibration of the wafer surface, the cleaning tool, such as a sponge or brush, may contact the wafer surface in a different manner, and as a result a uniform cleaning process may not be performed.
In particular, amid an increasing trend in a wafer diameter, a natural frequency of the wafer tends to decrease, and a rotational frequency of the wafer during processing approaches the natural frequency of the wafer. As a result, the vibration of the wafer surface can be enlarged due to a rotational speed of the wafer. Therefore, when multiple wafers are processed under the same conditions, the processing results may vary greatly depending on the structure and the shape of each wafer, thus causing a lowered yield of final products. This means that there is a case where the principle, in which the same processing conditions are required in order to obtain the same processing results, does not hold true.